In the semiconductor industry, a technology known as chemical-mechanical-polishing ("CMP") has been developed. In "CMP," a semiconductor wafer is place between a polishing pad and a wafer carrier. The wafer carrier is a mechanical device used to provide rotation of and downward pressure on the semiconductor wafer. In this manner, the semiconductor wafer is forcibly held and rotated against the polishing pad. As a result, material is abraded from the surface of the semiconductor wafer in contact with the polishing pad.
Slurry, a liquid chemical composition generally containing particulate, may be dispensed on the polishing pad. The slurry conventionally is chemically active to remove unwanted material from the semiconductor wafer. This chemical activity may be changed during CMP processing. The slurry particulate facilitates abrasion. Alternatively, the semiconductor wafer is polished without slurry by using a "fixed-abrasive polishing pad." A "fixed-abrasive polishing pad" has protrusions formed on it to facilitate abrasion. The semiconductor wafer is thus held and rotated against the protrusions on the fixed-abrasive polishing pad.
Aside from removing unwanted material, CMP may be used to planarize a surface of a semiconductor wafer. A uniform planar surface is important for subsequent lithographic processing of the semiconductor wafer, mainly due to depth of focus limitations of lithographic equipment. Accordingly, to achieve a substantially uniformly planar surface, it is important to apply pressure uniformly. A problem has been a lack of uniform pressure resulting in a non-uniform planar surface (e.g., a wedge-like shape).
One approach to achieve greater pressure uniformity has been to place an underpad below the polishing pad. The underpad has more give than the polishing pad, and thus it distributes downward force from the wafer carrier more uniformly. However, conditions, such as temperature, applied force, and friction, change during CMP, and an underpad is not able to compensate for these changes.
Another approach to achieve a more uniform downward force has been to use a wafer carrier having multiple bellows located between a wafer carrier housing and a wafer carrier base, as is described in U.S. Pat. No. 5,681,215 to Sherwood et al. As the semiconductor wafer is held between the polishing pad and the wafer carrier base, by pressurizing bellows against the wafer carrier housing, force is directed onto the wafer carrier base, which in turn pushes down on the semiconductor wafer. As each bellows may be separately pressurized, downward force is supposedly adjustable from location to location to improve uniformity; however, Sherwood et al is a mechanically complicated approach.
Therefore, it would be desirable to have a CMP apparatus that is less mechanically complicated than Sherwood et al, but allows for rigidity adjustment unlike present day underpads. Furthermore, it would be desirable to have a CMP apparatus that is a disposable item like an underpad.